According to the technical background, as illustrated with reference to FIG. 1, use is made of an illumination system 10 illuminating an imager 11.
Conventionally, the illumination system 10 comprises:                a source of illumination 100 with elliptical reflector;        a colour wheel 107;        a rectangular light guide 102; and        a system of several relay lenses 104 to 106.The source of illumination 100 illuminates with a light beam 101 the colour wheel 107 placed at the entry to the rectangular light guide 102 at the focal point of the elliptical reflector of the source 100. The rectangular light guide 102 is used to convert the circular cross section of the illuminating beam into a rectangular cross section and to make the beam spatially uniform.        
The exit of the light guide 102 is imaged on the imager 11 via the system of relay lenses, of which there are at least two, but of which there are often three or four, the illumination also being preferably telecentric.
If the imager 11 is of the DMD (Digital Micromirrors Device) type marketed by Texas Instruments®), a TIR prism 12 is placed between the illumination system 10 and the imager 11 in order to split the beams. The TIR prism 12 is not needed if the imager 11 is of the transmissive LCD (Liquid Crystal Display) type or is replaced by a PBS (Polarizing Beam Splitter) if the imager 11 is of the LCOS (Liquid Crystal On Silicon) type.
The main disadvantages with the technical background are the number of optical components (typically a guide and at least two relay lenses) and the numerical apertures needed to achieve good performance in terms of efficiency (from f/2.5 to f/3.0), imposing draconian conditions on a projection lens, particularly in its configuration as a shallow-depth projector.